This invention relates to a portable, audible beacon and, more specifically, to a portable, audible beacon which may be positioned at the exit of a building and used by firefighters and emergency service personnel as a directional guide to find their way out of the building in heavy smoke conditions.
Firefighters and other emergency service personnel are often required to operate inside buildings filled with heavy smoke. Under such conditions, the firefighters and emergency service personnel may become disoriented and lose their way to the exit of a building. Because of the limited air supply available to them or because of other dangerous conditions within the buildings, such disorientation is often a life-threatening matter for these personnel.
In the past, to assist guiding firefighters in exiting a building, some firefighting units have positioned a firefighter at the exit of a building to signal the firefighters inside. The firefighter at the exit either used a flashlight to provide a visual signal or shouted to provide an audible signal to the firefighters in the building.
This approach, however, has the following disadvantages. Primarily, it is an ineffective means of signaling. The use of a flashlight is ineffective because the light generated by a flashlight is unable to penetrate heavy smoke. Likewise, shouting is also ineffective because a person's shout is unable to be heard throughout a large building, especially amid the noise and chaos of a structural fire. Additionally, shouting is dangerous because a firefighter must remove his mask to shout effectively to his fellow firefighters, thus exposing himself to smoke inhalation and toxic gases. Moreover, posting a firefighter at the exit of a building is ineffective because the firefighter may be unable to remain at his station. The firefighter may be overcome by smoke or heat or may perceive a greater need for his help at another location. Further, posting a firefighter at the exit of a building is inefficient because it reduces the personnel available to fight a fire and to perform search and rescue operations. Finally, posting a firefighter at the exit of a building may be unnecessarily dangerous because it exposes the firefighter to the same dangerous conditions as the firefighters who have entered the building (such as building collapse, falling debris, radiant heat, and flames).
Another approach to assisting firefighters in smoke-filled conditions has been the use of personal alert safety systems or "PASS" devices. Such devices have been disclosed, for example, in U.S. Pat. Nos. 5,317,305 (Campman); 5,216,418 (Lenz); 4,926,159 (Bartlett); 4,468,656 (Clifford); and 4,090,185 (Patty). In general, PASS devices are portable devices carried or worn by firefighters which sound an alarm either automatically (under certain specified emergency conditions) or manually. The PASS alarm assists other firefighters in locating a firefighter in distress.
Unfortunately, the use of PASS devices focuses on bringing rescuers to a lost or injured firefighter. The PASS approach puts the rescuers in the same danger as the firefighter they are attempting to rescue. For this reason, an audible exit beacon is preferable to a PASS device because such a beacon could prevent a firefighter from becoming disoriented, lost and/or injured in the first place.
While technically a PASS device could be used as an audible exit beacon (by placing the PASS device at the exit of a building and initiating its alarm condition), a PASS device is not well-suited to performing the functions of an audible exit beacon. Because of the size and weight restrictions inherent in a device that must be conveniently carried by a firefighter on his person, PASS devices do not have sufficient power to project sound effectively throughout a large building. Furthermore, a device that is small enough to be conveniently carried by a firefighter on his person could easily become immersed in water or covered by a small amount of fallen debris. Under such conditions, it would be impossible for sound to emanate from the device, rendering it useless.
Another major problem typical of most PASS devices and the prior art in general has been the inability to combine power efficiency, auditory sensitivity, and directionality of sound in one device. Until now, audible beacon devices have not been able to combine all of these features because of the inherent design constraints arising from the physiology of human hearing.
One of the most important ways people determine the direction of distant sounds is by detecting the difference in the phase of the sound waves received by the two ears. This method becomes ineffective, however, when the wavelength of a sound approaches or becomes shorter than the distance of separation between the two ears. For adult humans, the directionality of a sound is usually lost when the frequency of the sound exceeds 2000 Hz (corresponding to a wavelength of under seven inches).
Within the range of normal human hearing (which is about 16 Hz to 20 KHz), the human ear is more sensitive to certain frequencies than to others. In particular, the sensitivity of the human ear is greatest at a frequency of about 2700 Hz, and it decreases as one gets further and further away from this peak frequency. The result of the human ear's varying sensitivity to different frequencies is that, at a given power level, some frequencies sound louder than others. For example, if a 2700 Hz sound and a 64 Hz sound are projected at the same power level, the 2700 Hz sound has an apparent loudness to a human ear of about 16 times greater than the 64 Hz sound. Conversely, to achieve the same apparent loudness of a 2700 Hz sound, a 64 Hz sound would require about 100,000 times more power than a 2700 Hz sound. Unfortunately, therefore, sounds that are directionally perceptible to humans are much more "power-hungry" than sounds that are not directionally perceptible.
Although not as constrained with respect to size and weight as PASS devices, an audible exit beacon must take these factors into consideration to be useful. The size and weight of an audible exit beacon cannot be so great as to render such a device non-portable. Portability of an audible exit beacon is essential to the firefighters who need to easily carry the beacon to the different buildings at which they are called to work. In addition, it is generally desirable to have an efficient power consumption so that an audible exit beacon can operate for as long a time as possible before its power source is exhausted. The longer the life of an audible exit beacon's power source, the less likely the possibility that the exit beacon will stop operating while firefighters are working. Such an interruption in operation would be, at the very least, an inconvenience and annoyance (since a firefighter would be diverted from his duties to replace the power source) and could possibly be dangerous under certain circumstances (for example, if the unit stopped operating while firefighters were attempting to escape from dangerous conditions).
One of the patent references cited earlier, Bartlett, discloses a device that attempts to overcome the trade-off between power efficiency, auditory sensitivity, and directionality of audible devices. Bartlett discloses a device that drives a resonant piezoelectric element by a frequency-modulated electrical input. The piezoelectric element resonates at a frequency of about 3000 Hz (a power-efficient frequency). Because of its resonant character, the piezoelectric element generates an amplitude-modulated sound output from the frequency-modulated electrical input. This amplitude-modulated sound output is characterized in Bartlett as a burst of 3000 Hz sound that lasts less than 1 millisecond and that is repeated about every 2 milliseconds. By using bursts of a power-efficient frequency, spaced apart so that the bursts are directionally perceptible, Bartlett's device purportedly achieves both power efficiency and directionality.
Bartlett's device, however, is not suitable for use as an audible exit beacon device. Bartlett's use of a piezoelectric device limits the power output of the device to a range that is unacceptable for audible exit beacons. Typically, portable piezoelectric devices operate in the milliwatt power range. For sound to be heard throughout a large building, an audible device must typically deliver tens of watts of power. Moreover, because of the resonating scheme used in Bartlett, one can not simply replace the piezoelectric device with a more powerful acoustic transducer, such as a loudspeaker, since such a transducer is not normally a resonator. Thus, the frequency modulation of the driving signal to a loudspeaker will not normally result in an amplitude modulation of an audible output signal. Finally, another disadvantage of Bartlett's scheme is that the device delivers output power during less than 50 percent of its cycle time.